[llvm] [EquivClasses] Fix signature of operator= (NFC) (PR #130140)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

I might be missing something, but as per my reading of https://en.cppreference.com/w/cpp/language/copy_assignment the current definition should be ok?

https://github.com/llvm/llvm-project/pull/130140